1. EECC551 - Shaaban #1 Lec # 10 Fall 2003 10-28-2003 Main Memory Main memory generally utilizes Dynamic RAM (DRAM), which use a single transistor to store a bit, but require a periodic data refresh by reading every row. Static RAM may be used for main memory if the added expense, low density, high power consumption, and complexity is feasible (e.g. Cray Vector Supercomputers). Main memory performance is affected by: –Memory latency: Affects cache miss penalty, M. Measured by: Access time: The time it takes between a memory access request is issued to main memory and the time the requested information is available to cache/CPU. Cycle time: The minimum time between requests to memory (greater than access time in DRAM to allow address lines to be stable) –Memory bandwidth: The maximum sustained data transfer rate between main memory and cache/CPU. (In Chapter 5.8 - 5.10)

2. EECC551 - Shaaban #2 Lec # 10 Fall 2003 10-28-2003 Four Key DRAM Timing Parameters t RAC : Minimum time from RAS (Row Access Strobe) line falling to the valid data output. –Usually quoted as the nominal speed of a DRAM chip –For a typical 64Mb DRAM t RAC = 60 ns t RC : Minimum time from the start of one row access to the start of the next (memory cycle time). –t RC = 110 ns for a 64Mbit DRAM with a t RAC of 60 ns t CAC : minimum time from CAS (Column Access Strobe) line falling to valid data output. –12 ns for a 64Mbit DRAM with a t RAC of 60 ns t PC : minimum time from the start of one column access to the start of the next. –About 25 ns for a 64Mbit DRAM with a t RAC of 60 ns

3. EECC551 - Shaaban #3 Lec # 10 Fall 2003 10-28-2003 Simplified DRAM Speed Parameters Row Access Strobe (RAS)Time: (similar to t RAC ) –Minimum time from RAS (Row Access Strobe) line falling to the first valid data output. –A major component of memory latency. –Only improves ~ 5% every year. Column Access Strobe (CAS) Time/data transfer time: (similar to t CAC ) –The minimum time required to read additional data by changing column address while keeping the same row address. – Along with memory bus width, determines peak memory bandwidth.

4. EECC551 - Shaaban #4 Lec # 10 Fall 2003 10-28-2003 DRAM Generations YearSize RAS (ns) CAS (ns) Cycle Time Memory Type 1980 64 Kb 150-18075 250 nsPage Mode 1983 256 Kb 120-15050 220 ns Page Mode 1986 1 Mb 100-12025 190 ns 1989 4 Mb 80-10020 165 ns Fast Page Mode 1992 16 Mb 60-8015 120 ns EDO 1996 64 Mb 50-7012 110 ns PC66 SDRAM 1998 128 Mb 50-7010 100 ns PC100 SDRAM 2000 256 Mb 45-657 90 ns PC133 SDRAM 2002 512 Mb 40-605 80 ns PC2700 DDR SDRAM 8000:1 15:1 3:1 (Capacity) (~bandwidth) (Latency)

5. EECC551 - Shaaban #5 Lec # 10 Fall 2003 10-28-2003 Simplified Asynchronous DRAM Read Timing Source: http://arstechnica.com/paedia/r/ram_guide/ram_guide.part2-1.html (late 70s)

6. EECC551 - Shaaban #6 Lec # 10 Fall 2003 10-28-2003 Page Mode DRAM: Motivation

7. EECC551 - Shaaban #7 Lec # 10 Fall 2003 10-28-2003 Page Mode DRAM: Operation

8. EECC551 - Shaaban #8 Lec # 10 Fall 2003 10-28-2003 Simplified Asynchronous Fast Page Mode (FPM) DRAM Read Timing Typical timing at 66 MHz : 5-3-3-3 For bus width = 64 bits = 8 bytes cache block size = 32 bytes It takes = 5+3+3+3 = 14 memory cycles or 15 ns x 14 = 210 ns to read 32 byte block Read Miss penalty for CPU running at 1 GHz =M = 15 x 14 = 210 CPU cycles FPM DRAM speed rated using tRAC ~ 50-70ns (late 80s)

9. EECC551 - Shaaban #9 Lec # 10 Fall 2003 10-28-2003 Extended Data Out DRAM operates in a similar fashion to Fast Page Mode DRAM except the data from one read is on the output pins at the same time the column address for the next read is being latched in. Simplified Asynchronous Extended Data Out (EDO) DRAM Read Timing Source: http://arstechnica.com/paedia/r/ram_guide/ram_guide.part2-1.html Typical timing at 66 MHz : 5-2-2-2 For bus width = 64 bits = 8 bytes Max. Bandwidth = 8 x 66 / 2 = 264 Mbytes/sec It takes = 5+2+2+2 = 11 memory cycles or 15 ns x 11 = 165 ns to read 32 byte cache block Minimum Read Miss penalty for CPU running at 1 GHz = M = 11 x 15 = 165 CPU cycles EDO DRAM speed rated using tRAC ~ 40-60ns (early 90s)

10. EECC551 - Shaaban #10 Lec # 10 Fall 2003 10-28-2003 Synchronous DRAM Interface Characteristics Summary # of Banks per DRAM Chip 2 4 4? 32 SDRAMDDR SDRAMRAMbus (Late 2003) (Similar to PC3200)

11. EECC551 - Shaaban #11 Lec # 10 Fall 2003 10-28-2003 Synchronous Dynamic RAM, SDRAM Organization DDR SDRAM organization is similar but four banks are used in each DDR SDRAM chip instead of two. Data transfer on both rising and falling edges of the clock DDR SDRAM rated by maximum memory bandwidth PC3200 = 8 bytes x 200 MHz x 2 = 3200 Mbytes/sec (mid 90s) (late 90s - current) SDRAM speed is rated at max. clock speed supported: 100MHZ = PC100 133MHZ = PC133

12. EECC551 - Shaaban #12 Lec # 10 Fall 2003 10-28-2003 Simplified SDRAM Read Timing Typical timing at 133 MHz (PC133 SDRAM) : 5-1-1-1 For bus width = 64 bits = 8 bytes Max. Bandwidth = 133 x 8 = 1064 Mbytes/sec It takes = 5+1+1+1 = 8 memory cycles or 7.5 ns x 8 = 60 ns to read 32 byte cache block Minimum Read Miss penalty for CPU running at 1 GHz = M = 7.5 x 8 = 60 CPU cycles (mid 90s)

13. EECC551 - Shaaban #13 Lec # 10 Fall 2003 10-28-2003 Memory Bandwidth Improvement Techniques Wider Main Memory: Memory width is increased to a number of words (usually the size of a cache block).  Memory bandwidth is proportional to memory width. e.g Doubling the width of cache and memory doubles memory bandwidth Simple Interleaved Memory: Memory is organized as a number of banks each one word wide. –Simultaneous multiple word memory reads or writes are accomplished by sending memory addresses to several memory banks at once. –Interleaving factor: Refers to the mapping of memory addressees to memory banks. e.g. using 4 banks, bank 0 has all words whose address is: (word address mod) 4 = 0

14. EECC551 - Shaaban #14 Lec # 10 Fall 2003 10-28-2003 Three examples of bus width, memory width, and memory interleaving to achieve higher memory bandwidth Narrow bus and cache with interleaved memory Wider memory, bus and cache Simplest design: Everything is the width of one word

15. EECC551 - Shaaban #15 Lec # 10 Fall 2003 10-28-2003 Memory Interleaving Number of banks  Number of cycles to access word in a bank

16. EECC551 - Shaaban #16 Lec # 10 Fall 2003 10-28-2003 Memory Width, Interleaving: Performance Example Given the following system parameters with single unified cache level L 1 (ignoring write policy): Block size= 1 word Memory bus width= 1 word Miss rate =3% M = Miss penalty = 32 cycles (4 cycles to send address 24 cycles access time, 4 cycles to send a word) Memory access/instruction = 1.2 CPI execution (ignoring cache misses) = 2 Miss rate (block size = 2 word = 8 bytes ) = 2% Miss rate (block size = 4 words = 16 bytes) = 1% The CPI of the base machine with 1-word blocks = 2 + (1.2 x 0.03 x 32) = 3.15 Increasing the block size to two words gives the following CPI: 32-bit bus and memory, no interleaving, M = 2 x 32 = 64 cycles CPI = 2 + (1.2 x.02 x 64) = 3.54 32-bit bus and memory, interleaved, M = 4 + 24 + 8 = 36 cycles CPI = 2 + (1.2 x.02 x 36) = 2.86 64-bit bus and memory, no interleaving, M = 32 cycles CPI = 2 + (1.2 x 0.02 x 32) = 2.77 Increasing the block size to four words; resulting CPI: 32-bit bus and memory, no interleaving, M = 4 x 32 = 128 cycles CPI = 2 + (1.2 x 0.01 x 128) = 3.54 32-bit bus and memory, interleaved, M = 4 + 24 + 16 = 44 cycles CPI = 2 + (1.2 x 0.01 x 44) = 2.53 64-bit bus and memory, no interleaving, M = 2 x 32 = 64 cycles CPI = 2 + (1.2 x 0.01 x 64) = 2.77 64-bit bus and memory, interleaved, M = 4 + 24 + 8 = 36 cycles CPI = 2 + (1.2 x 0.01 x 36) = 2.43 128-bit bus and memory, no interleaving, M = 32 cycles CPI = 2 + (1.2 x 0.01 x 32) = 2.38

17. EECC551 - Shaaban #17 Lec # 10 Fall 2003 10-28-2003 Three-Level Cache Example CPU with CPI execution = 1.1 running at clock rate = 500 MHz 1.3 memory accesses per instruction. L 1 cache operates at 500 MHz with a miss rate of 5% L 2 cache operates at 250 MHz with a local miss rate 40%, (T 2 = 2 cycles) L 3 cache operates at 100 MHz with a local miss rate 50%, (T 3 = 5 cycles) Memory access penalty, M= 100 cycles. Find CPI. With No Cache, CPI = 1.1 + 1.3 x 100 = 131.1 With single L 1, CPI = 1.1 + 1.3 x.05 x 100 = 7.6 With L1, L2 CPI = 1.1 + 1.3 x (.05 x.6 x 2 +.05 x.4 x 100) = 3.778 CPI = CPI execution + Mem Stall cycles per instruction Mem Stall cycles per instruction = Mem accesses per instruction x Stall cycles per access Stall cycles per memory access = (1-H1) x H2 x T2 + (1-H1) x (1-H2) x H3 x T3 + (1-H1)(1-H2) (1-H3)x M =.05 x.6 x 2 +.05 x.4 x.5 x 5 +.05 x.4 x.5 x 100 =.097 +.0075 +.00225 = 1.11 CPI = 1.1 + 1.3 x 1.11 = 2.54 Speedup compared to L1 only = 7.6/2.54 = 3 Speedup compared to L1, L2 = 3.778/2.54 = 1.49 Repeated from lecture 8

18. EECC551 - Shaaban #18 Lec # 10 Fall 2003 10-28-2003 3-Level (All Unified) Cache Performance Memory Access Tree (Ignoring Write Policy) CPU Stall Cycles Per Memory Access CPU Memory Access L1 Miss: % = (1-H1) L1 Hit: Stalls= H1 x 0 = 0 ( No Stall) L2 Miss: % = (1-H1)(1-H2) L2 Hit: (1-H1) x H2 x T2 Stall cycles per memory access = (1-H1) x H2 x T2 + (1-H1) x (1-H2) x H3 x T3 + (1-H1)(1-H2) (1-H3)x M AMAT = 1 + Stall cycles per memory access L3 Miss: (1-H1)(1-H2)(1-H3) x M L3 Hit: (1-H1) x (1-H2) x H3 x T3 L1L1 L3L3 L2L2 Repeated from lecture 8 To Main Memory

19. EECC551 - Shaaban #19 Lec # 10 Fall 2003 10-28-2003 Program Steady-State Main Memory Bandwidth-Usage Example In the previous example with three levels of cache (all unified, ignore write policy) CPU with CPI execution = 1.1 running at clock rate = 500 MHz 1.3 memory accesses per instruction. L 1 cache operates at 500 MHZ with a miss rate of 5% L 2 cache operates at 250 MHZ with a local miss rate 40%, (T 2 = 2 cycles) L 3 cache operates at 100 MHZ with a local miss rate 50%, (T 3 = 5 cycles) Memory access penalty, M= 100 cycles (to deliver 32 bytes to CPU) We found the CPI: With No Cache, CPI = 1.1 + 1.3 x 100 = 131.1 With single L 1, CPI = 1.1 + 1.3 x.05 x 100 = 7.6 With L1, L2 CPI = 1.1 + 1.3 x (.05 x.6 x 2 +.05 x.4 x 100) = 3.778 With L1, L2, L3 CPI = 1.1 + 1.3 x 1.11 = 2.54 Assuming: instruction size = data size = 4 bytes, all cache blocks are 32 bytes For each of the three cases with cache: What is the total number of memory accesses generated by the CPU per second? What is the percentage of these memory accesses satisfied by main memory? Percentage of main memory bandwidth used by the CPU?

20. EECC551 - Shaaban #20 Lec # 10 Fall 2003 10-28-2003 Memory requires 100 CPU cycles = 200 ns to deliver 32 bytes, thus total main memory bandwidth = 32 bytes / (200 ns) = 160 x 10 6 bytes/sec The total number of memory accesses generated by the CPU per second = (memory access/instruction) x clock rate / CPI = 1.3 x 500 x 10 6 / CPI = 650 x 10 6 / CPI –With single L1 = 650 x 10 6 / 7.6 = 85 x 10 6 accesses/sec –With L1, L2 = 650 x 10 6 / 3.778 = 172 x 10 6 accesses/sec –With L1, L2, L3 = 650 x 10 6 / 2.54 = 255 x 10 6 accesses/sec The percentage of these memory accesses satisfied by main memory: –With single L1 = L1 miss rate = 5% –With L1, L2 = L1 miss rate x L2 miss rate =.05 x.4 = 2% –with L1, L2, L3 = L1 miss rate x L2 miss rate x L3 miss rate =.05 x.4 x. 5 = 1% Memory Bandwidth used –With single L1 = 32 bytes x 85x10 6 accesses/sec x.05 = 136 x10 6 bytes/sec or 136/160 = 85 % of total memory bandwidth –With L1, L2 = 32 bytes x 172 x10 6 accesses/sec x.02 = 110 x10 6 bytes/sec or 110/160 = 69 % of total memory bandwidth –With L1, L2, L3 = 32 bytes x 255 x10 6 accesses/sec x.01 = 82 x10 6 bytes/sec or 82/160 = 51 % of total memory bandwidth Program Steady-State Main Memory Bandwidth-Usage Example

21. EECC551 - Shaaban #21 Lec # 10 Fall 2003 10-28-2003 Computer System Components SDRAM PC100/PC133 100-133MHZ 64-128 bits wide 2-way inteleaved ~ 900 MBYTES/SEC )64bit) Double Date Rate (DDR) SDRAM PC3200 200 MHZ DDR 64-128 bits wide 4-way interleaved ~3.2 GBYTES/SEC (one 64bit channel) ~6.4 GBYTES/SEC (two 64bit channels) RAMbus DRAM (RDRAM) 400MHZ DDR 16 bits wide (32 banks) ~ 1.6 GBYTES/SEC CPU Caches System Bus I/O Devices: Memory Controllers adapters Disks Displays Keyboards Networks NICs I/O Buses Memory Controller Example: PCI, 33-66MHZ 32-64 bits wide 133-528 MBYTES/SEC CPU Core 1 GHz - 3.0 GHz 4-way Superscaler RISC or RISC-core (x86): Deep Instruction Pipelines Dynamic scheduling Multiple FP, integer FUs Dynamic branch prediction Hardware speculation L1 L2 L3 Memory Bus All Non-blocking caches L1 16-128K 1-2 way set associative (on chip), separate or unified L2 256K- 2M 4-32 way set associative (on chip) unified L3 2-16M 8-32 way set associative (off or on chip) unified Examples: Alpha, AMD K7: EV6, 200-400 MHZ Intel PII, PIII: GTL+ 133 MHZ Intel P4 800 MHZ North Bridge South Bridge Chipset

22. EECC551 - Shaaban #22 Lec # 10 Fall 2003 10-28-2003 X86 CPU Cache/Memory Performance Example AMD Athlon T-Bird Vs. Intel PIII AMD Athlon T-Bird 1GHZ L1: 64K INST, 64K DATA (3 cycle latency), both 2-way L2: 256K 16-way 64 bit bus Latency: 7 cycles L1,L2 on-chip Intel PIII GHZ L1: 16K INST, 16K DATA (3 cycle latency), both 2-way L2: 256K 8-way 256 bit, Latency: 7 cycles L1,L2 on-chip (32 byte blocks) Main Memory: PC2100 133MHZ DDR SDRAM 64bit Peak bandwidth: 2100 MB/s Latency Range: 19ns - 64ns PC133 133MHZ SDRAM 64bit Peak bandwidth: 1000 MB/s Latency Range: 25ns - 80ns PC800 Rambus DRDRAM 400 MHZ DDR 16-bit Peak bandwidth: 1600 MB/s (1 channel) Latency Range: 35ns - 80ns Intel 840 uses two PC800 channels Source: http://www1.anandtech.com/showdoc.html?i=1344&p=9 L1 Hit L1 Miss L2 Hit L1 Miss L2 Miss 64K 320K

23. EECC551 - Shaaban #23 Lec # 10 Fall 2003 10-28-2003 X86 CPU Cache/Memory Performance Example: AMD Athlon T-Bird Vs. Intel PIII, Vs. P4 AMD Athlon T-Bird 1GHZ L1: 64K INST, 64K DATA (3 cycle latency), both 2-way L2: 256K 16-way 64 bit bus Latency: 7 cycles L1,L2 on-chip Intel PIII 1 GHZ L1: 16K INST, 16K DATA (3 cycle latency) both 2-way 32 byte blocks L2: 256K 8-way 256 bit bus, Latency: 7 cycles L1,L2 on-chip Intel P 4, 1.5 GHZ L1: 8K DATA (2 cycle latency) 4-way 64 byte blocks 96KB Execution Trace Cache L2: 256K 8-way 256 bit bus, 128 byte blocks Latency: 7 cycles L1,L2 on-chip Source: http://www1.anandtech.com/showdoc.html?i=1360&p=15

24. EECC551 - Shaaban #24 Lec # 10 Fall 2003 10-28-2003 X86 CPU Cache/Memory Performance Example: AMD Athlon T-Bird Vs. Duron AMD Athlon T-Bird 750MHZ-1GHZ L1: 64K INST, 64K DATA, both 2-way L2: 256K 16-way 64 bit Latency: 7 cycles L1,L2 on-chip AMD Athlon Duron 750MHZ-1GHZ L1: 64K INST, 64K DATA both 2-way L2: 64K 16-way 64 bit Latency: 7 cycles L1,L2 on-chip Source: http://www1.anandtech.com/showdoc.html?i=1345&p=10 Memory: PC2100 133MHZ DDR SDRAM 64bit Peak bandwidth: 2100 MB/s PC1600 100MHZ DDR SDRAM 64bit Peak bandwidth: 1600 MB/s L1 Miss L2 Miss L1 Hit

25. EECC551 - Shaaban #25 Lec # 10 Fall 2003 10-28-2003 A Typical Memory Hierarchy Control Datapath Virtual Memory, Secondary Storage (Disk) Processor Registers Main Memory (DRAM) Second Level Cache (SRAM) L 2 1s 10,000,000s (10s ms) Speed (ns): 10s100s GsSize (bytes):KsMs Tertiary Storage (Tape) 10,000,000,000s (10s sec) Ts On-Chip Level One Cache L 1 Larger Capacity Faster

26. EECC551 - Shaaban #26 Lec # 10 Fall 2003 10-28-2003 Virtual Memory Virtual memory controls two levels of the memory hierarchy: Main memory (DRAM). Mass storage (usually magnetic disks). Main memory is divided into blocks allocated to different running processes in the system: Fixed size blocks: Pages (size 4k to 64k bytes). Variable size blocks: Segments (largest size 2 16 up to 2 32 bytes ). At any given time, for any running process, a portion of its data/code is loaded in main memory while the rest is available only in mass storage. A program code/data block needed for process execution and not present in main memory result in a page fault (address fault) and the block has to be loaded into main main memory from disk (demand paging). A program can be run in any location in main memory or disk by using a relocation mechanism controlled by the operating system which maps the address from virtual address space (logical program address) to physical address space (main memory, disk).

27. EECC551 - Shaaban #27 Lec # 10 Fall 2003 10-28-2003 –Illusion of having more physical main memory –Allows program relocation –Protection from illegal memory access Virtual Memory Benefits

28. EECC551 - Shaaban #28 Lec # 10 Fall 2003 10-28-2003 Paging Versus Segmentation

29. EECC551 - Shaaban #29 Lec # 10 Fall 2003 10-28-2003 Physical location of blocks A, B, C Contiguous virtual address space of a program Virtual Physical Address Translation Virtual  Physical Address Translation Page Fault: D in Disk (not allocated in main memory)

30. EECC551 - Shaaban #30 Lec # 10 Fall 2003 10-28-2003 Mapping Virtual Addresses to Physical Addresses Using A Page Table

31. EECC551 - Shaaban #31 Lec # 10 Fall 2003 10-28-2003 Virtual Address Translation

32. EECC551 - Shaaban #32 Lec # 10 Fall 2003 10-28-2003 Page Table Organization Two memory accesses needed: First to page table. Second to item.

33. EECC551 - Shaaban #33 Lec # 10 Fall 2003 10-28-2003 Typical Parameter Range For Cache & Virtual Memory

34. EECC551 - Shaaban #34 Lec # 10 Fall 2003 10-28-2003 Virtual Memory Issues/Strategies Main memory block placement: Fully associative placement is used to lower the miss rate. Block replacement: The least recently used (LRU) block is replaced when a new block is brought into main memory from disk. Write strategy: Write back is used and only those pages changed in main memory are written to disk (dirty bit scheme is used). To locate blocks in main memory a page table is utilized. The page table is indexed by the virtual page number and contains the physical address of the block. –In paging: Offset is concatenated to this physical page address. –In segmentation: Offset is added to the physical segment address. To limit the size of the page table to the number of physical pages in main memory a hashing scheme is used. Utilizing address locality, a translation look-aside buffer (TLB) is usually used to cache recent address translations and prevent a second memory access to read the page table.

35. EECC551 - Shaaban #35 Lec # 10 Fall 2003 10-28-2003 Speeding Up Address Translation: Translation Lookaside Buffer (TLB) TLB: A small on-chip fully-associative cache used for address translations. If a virtual address is found in TLB (a TLB hit), the page table in main memory is not accessed.

36. EECC551 - Shaaban #36 Lec # 10 Fall 2003 10-28-2003 Operation of The Alpha 21264 Data TLB (DTLB) During Address Translation Virtual address DTLB = 128 entries DTLB = 128 entries ProtectionPermissions Valid bit Address Space Number (ASN) Identifies process similar to PID (no need to flush TLB on context switch)

37. EECC551 - Shaaban #37 Lec # 10 Fall 2003 10-28-2003 A Memory Hierarchy With TLB & Two Levels of Cache TLB: Direct Mapped 256 entries L1 direct mapped 8KB L2 direct mapped 4MB Virtual address 64 bits Physical address 41 bits

38. EECC551 - Shaaban #38 Lec # 10 Fall 2003 10-28-2003 TLB & Cache Operation Cache is physically-addressed TLB Operation

39. EECC551 - Shaaban #39 Lec # 10 Fall 2003 10-28-2003 CPU Performance with Real TLBs When a real TLB is used with a TLB miss rate and a TLB miss penalty is used: CPI = CPI execution + mem stalls per instruction + TLB stalls per instruction Where: Mem Stalls per instruction = Mem accesses per instruction x mem stalls per access Similarly: TLB Stalls per instruction = Mem accesses per instruction x TLB stalls per access TLB stalls per access = TLB miss rate x TLB miss penalty Example: Given: CPI execution = 1.3 Mem accesses per instruction = 1.4 Mem stalls per access =.5 TLB miss rate =.3% TLB miss penalty = 30 cycles What is the resulting CPU CPI? Mem Stalls per instruction = 1.4 x.5 =.7 cycles/instruction TLB stalls per instruction = 1.4 x (TLB miss rate x TLB miss penalty) = 1.4 x.003 x 30 =.126 cycles/instruction CPI = 1. 3 +.7 +.126 = 2.126

40. EECC551 - Shaaban #40 Lec # 10 Fall 2003 10-28-2003 Event Combinations of Cache, TLB, Virtual Memory CacheTLB Virtual Possible? When? Memory MissHit HitPossible, no need to check page table HitMiss HitTLB miss, found in page table MissMiss HitTLB miss, cache miss MissMiss MissPage fault MissHit MissImpossible, cannot be in TLB if not in memory HitHit MissImpossible, cannot be in TLB or cache if not in memory HitMiss MissImpossible, cannot be in cache if not in memory